Thermoregulation During Extended Exercise in the Heat: Comparisons of Fluid Volume and Temperature

Objective This study aimed to determine the physiological and thermoregulatory responses of individuals exercising in the heat (US military red flag conditions, wet-bulb globe temperature 31.5–32.2ºC) while consuming varied volumes of ambient temperature water and ice slurry. Methods Participants (N = 12) walked on a treadmill for 3 hours at approximately 40% peak aerobic capacity in a hot environment while consuming ambient temperature (35.5°C) water (W), ice slurry (0°C, two-thirds shaved ice and one-third water) at a ratio of 2 g·kg^-1 body mass every 10 minutes (FS), and reduced volume ice slurry as described at a rate of 1 g·kg^-1 body mass every 10 minutes (HS). Trials were completed at least 14 days apart, in a randomized, repeated measures design. Results Percent body weight loss was higher during the HS trial (1.8 ± 0.01%) compared with FS (0.5 ± 0.01%; P < .001) and W (0.6 ± 0.01%; P < .001). Mean rectal temperature at 3 hours was lower during FS (37.8 ± 0.7°C) compared with HS (38.1 ± 0.8°C) and W (38.2 ± 0.8°C) (P = .04 vs HS, and P = .005 vs W, main effect for trial). No differences were found in rectal temperature between HS and W. Heart rate was lower at the end of the third hour during FS (141 ± 10 beats/min) compared with HS (157 ± 19 beats/min) and W (154 ± 18 beats/min) (P = .001 and P = .007, respectively, time × trial interaction). There were no differences in heart rate between HS and W. Conclusions The temperature of consumed fluids may be as important as the volume for the management of thermoregulation and other physiological responses for extended work in hot environments.